Variance component analysis of skin and weight data for sheep subjected to rapid inbreeding

A variance component analysis was carried out on data from a 20-year experiment in the rapid inbreeding of purebred and crossbred lines of three hill breeds of sheep. Parent offspring matings were made over several generations to produce inbreeding coefficients in lambs of up to 0.59. The traits chosen for analysis were the live weights at 24 and 78 weeks of age and th ratio of the densities of secondary and primary skin follicles. A complete model of intralocus allelic effects was carried out with both additive genetic variance and dominance variance. The latter was partitioned into components arising from loci which were homozygous by descent and those that were not. Inbreeding depression was fitted as a covariate. This model has not been attempted previously in livestock populations. Crossbred animals were found to exhibit more dominance variance than purebred animals. Though partitioning of the dominance variance was possible in some of the data sets considered, estimation of the novel quadratic components was difficult and provided little evidence of homozygous dominance variance as distinguished from the familiar random dominance variance (that arising in randomly mated populations). A pooled dominance model is proposed in which inbred dominance effects have the same variance as random dominance effects. For live weight the results suggested that the genetic architecture involved many loci with deleterious recessive alleles, but for the ratio of follicle density there was no clear explanation for the results observed. (C) Inra/Elsevier, Paris.